Energy companies have boosted profits to more than £3billion in the last five years while hiking customer prices, unions claimed yesterday.

Unite, the UK's biggest union, compiled a "dossier of disgrace" showing gas and electricity profits among the biggest energy suppliers rose by 538 per cent since 2003.

And union chiefs turned up the heat on ministers, saying the "obscene" profits - equivalent to £95 a second - make the case for a windfall tax "compelling".

They claim the companies spend just £50million a year combating fuel poverty.

By contrast the companies paid £1.6billion to shareholders last year, up almost a fifth on the previous 12 months.

Unite joint chief Derek Simpson said: "Winter is approaching and low-paid families will be forced to switch their heating off because they won't be able to afford the bill.

"Heartless excuses from energy firms for not fulfilling their social responsibilities have been swallowed hook, line and sinker by the Government. It's time for action, the situation is dire."

The Government estimates 2.5 million families are in fuel poverty, with bills at 10 per cent of income.

But Energywatch believes it is more than four million as every 10 per cent price increase plunges another 400,000 into that bracket.

Gas prices have doubled in price since 2000 and electricity has soared 61 per cent.

Unite, Labour's biggest financial backer, is hoping its "dossier of disgrace" will pile pressure on Chancellor Alistair Darling, who is due to deliver his keynote speech at the TUC Congress in Brighton today.

Tuc general secretary Brendan Barber has urged ministers to use some of the windfall cash to increase winter fuel payments to the poor and elderly from £150 to £400.

Tony Woodley, joint leader of Unite, said: "The greedy oil companies have made tens of billions and in the next four years they will make an extra £15billion from the British public.

"The Government must intervene now. Our case for a windfall tax is compelling. It is morally right. I say to the Government it's time to do the right thing and protect the most vulnerable."

The dossier was published on the same day a report revealed almost a quarter of the British population will be in fuel poverty by the end of next year.

The National Housing Federation estimates 5.7million households will spend at least 10 per cent of income on power as average bills hit £1,406, up from £676 in 2005.